Human Ancestors May Have Evolved to Walk Upright in Trees

When human ancestors evolved to walk upright, they may have done so in trees, suggests new research published Wednesday in the journal Science Advances.

The findings contrast with a prevailing theory about human bipedalism—walking on two feet—known as the savanna hypothesis. This theory posits that human ancestors began walking upright when forests retreated and the landscape transformed into more open, savanna-like areas. As a result, hominins began spending less time in trees and more time on the ground, traveling and foraging for food, per the hypothesis. The ability to walk on two feet would have been helpful for seeing over tall grass, as well as for carrying objects, scientists suggested.

However, the new research proposes human ancestors instead may have begun walking upright to move around tree branches in search of food.

“The retreat of forests in the late Miocene-Pliocene era around five million years ago and the more open savanna habitats were, in fact, not a catalyst for the evolution of bipedalism,” study co-author Alex Piel, a biological anthropologist at University College London in England, says in a statement. “Instead, trees probably remained essential to its evolution—with the search for food-producing trees likely a driver of this trait.”

Scientists reached this conclusion after studying wild chimpanzees—which are humans’ closest living relatives—in the Issa Valley of western Tanzania. The chimpanzees’ environment is what’s known as a savanna-mosaic, which features both open landscapes with few trees and densely forested areas with lots of trees. Scientists were keen to study these chimpanzees, because this environment closely resembles that of early human ancestors.

“While savanna ecologies are often centered in conversations on bipedalism, rarely do we have the opportunity to learn from chimpanzees living in such habitats,” says Alyssa Crittenden, an anthropologist at the University of Nevada Las Vegas, to Popular Science’s Jocelyn Solis-Moreira. “This important study provides support for the hypothesis that hominin bipedalism likely evolved within an arboreal context and persisted long after hominids began living in and utilizing more open landscapes with less vegetation.”

During their 15-month experiment, the researchers observed 13 adult chimpanzees and made note of their movements, such as whether the animals were hanging, walking or climbing. In total, they recorded more than 13,700 observations, including 2,850 movement events. They also noted where the chimpanzees were during each movement—in trees or on land—as well as whether they were in densely forested areas or more open regions. They then compared their findings to the behaviors of other chimpanzees that live exclusively in dense forests.

The researchers had wondered if the Issa Valley chimpanzees would spend more time on the ground, because their habitat included open areas. But after analyzing the data, they found these chimps spent just as much time in the trees as their forest-dwelling counterparts did.

“Issa chimpanzees were no more terrestrial in woodland vegetation than chimpanzees living in forest habitats, suggesting that it is not a simple rule of less trees means more time on the ground,” the researchers write in the paper.

Scientists also expected to see the chimpanzees more frequently walking upright in the open areas of their habitat. But the data showed otherwise: More than 85 percent of the chimpanzees’ bipedalism events occurred in the trees.

The researchers hypothesize that the animals walked upright in the trees to reach food in the branches, as well as to stay safe from predators in the ground habitat. During the rainy season in the Issa Valley, the grasses in the open areas can grow to reach more than six feet tall, which would make the chimpanzees easier prey for stealthy land-dwelling predators that might sneak up on them, reports CNN’s Jack Guy.

The new theory still needs to be tested against the fossil record, and it brings up more questions than answers. For example, the findings don’t shed any light on why early humans, rather than apes, first began walking on two feet. Humans and chimpanzees shared a common ancestor roughly 6 to 8 million years ago but evolved very differently from that point forward, per Smithsonian’s National Museum of Natural History.

And the theory also doesn’t explain how some animals that do not live in trees, such as kangaroos, evolved to walk on two feet. Moving forward, the scientists also hope to conduct further research to understand more about why the Issa Valley chimpanzees spend so much time in the canopy.

“Bipedalism is a defining feature of the human lineage and is the first thing to separate our fossil ancestors from other apes,” says study co-author Rhianna Drummond-Clarke, a biological anthropologist at the University of Kent in England, to Popular Science. “Understanding why it evolved is thus key to understanding what made us human.”

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Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.